There has been a long-standing need in the semiconductor industry to characterize the properties of a semiconductor device. As dimensions of devices in this industry are diminishing, increasingly sensitive metrology tools and analysis techniques are required for measurement of the properties of these devices, in particular, devices comprising a stack of thin films on a semiconductor substrate.
Optical metrology tools used for such measurements are typically ellipsometry and reflectometry based tools. Reflectometry based tools typically measure changes in the magnitude of radiation reflected/transmitted from/through the sample, and ellipsometry based tools typically measure changes of the polarization state of radiation after interacting with the sample.
Measured optical data, indicative of the detected radiation (reflected and/or transmitted), can be analyzed to derive information regarding the optical constants of materials included in the sample, as well as the layer parameters, such as thickness and geometrical parameters of patterns (including critical dimension (CD), line spacing, line width, wall depth, and wall profile).
Examples of the measurement techniques of the kind specified are disclosed in “Simultaneous Measurement of Six Layers in a Silicon on Insulator Film Stack Using Spectrophotometry and Beam profile Reflectometry”, J. M. Leng at al., J. Appl. Physics, 81(8), 15 Apr. 1997, pp. 3570-3578; US 2006/0167651; U.S. Pat. No. 7,259,850; U.S. Pat. No. 5,999,267; and U.S. Pat. No. 6,091,485.
Also known are methods based on selective material removal from the full multi-layer structure, which are disclosed in U.S. Pat. No. 7,289,234 and U.S. Pat. No. 7,019,850 both assigned to the assignee of the present application.